Telecentric technology refers to an optical design in which the exit pupil position is disposed at an infinite point to ensure that the chief ray of the optical system of the Fθ lens is parallel to the optical axis. That is, all rays focused at the focal point on the image surface have the same aperture angle, and an image of an object of substantially fixed size can be obtained on the image surface. Since it is required that all the exit pupil chief rays must be parallel to each other, the lens is required to have a larger optical aperture when the image has a larger format; that is, the larger the format, the larger the lens is. The volume of the lens is proportional to the optical aperture D or D3, so the Fθ lens of large format is rarely applied in practice.
For a conventional telecentric system, the entrance pupil position is always disposed at the front focus of the system to allow the chief ray of the focused ray to be perpendicular to the focal plane. When the lens has a long focal length, the entrance pupil position is at a distant point due to the far distance of the focal point. This not only increases the whole volume of the optical system of the Fθ lens, but also increases the difficulty of correcting the aberrations of the optical system of the Fθ lens.